1. a depraved, unprincipled, or wicked person: a drunken reprobate.
2.a person rejected by God and beyond hope of salvation.
This is a term I had always thought of as fairly prevalent. Calling someone a drunken reprobate was par for the course plenty of old movies. I had no notion the word didn’t enjoy widespread use in the common vernacular.
When I’ve tried to explain what a reprobate is, I’ve said that it’s essentially interchangeable with being a scoundrel. But upon further reflection it occurs to me that to call someone a scoundrel implies an impish or playful element to their behavior, whereas to my mind, a reprobate is someone who is willfully wicked and relishing their misdeeds; unapologetically wayward. 
In my own usage, I most often apply it to licentiousness rather than intoxication. Which, while utterly in the scope of the proper definition, isn’t the reference most commonly made in common conversation, so that can cause a cocked head here and there as well. 
My commitment to helping enhance understanding is such that I wonder; perhaps I could clarify matters by pointed demonstration? Make a t-shirt that says “Reprobate” and then behave accordingly?